To control a motor at high speeds with high precision, it is necessary to generate rotating magnetic flux in accordance with the magnetic pole position of the rotor. However, using a position sensor to detect the magnetic pole position involves various problems, such as high cost, vulnerability to vibration and heat, increase in motor size, increased wiring, and restrictions on wiring length.
Heretofore, such problems have prompted the development of methods to detect the magnetic pole position without the use of a position sensor, and a method of estimating the magnetic pole position of a rotor by using the induced voltage caused by the magnetic flux of the permanent magnet during the rotation (sensorless vector control) is widely known. This method has drawbacks in that it is difficult to detect or estimate the induced voltage at low speeds where the induced voltage is small, degradation in precision with which the magnetic pole position of the rotor is detected, and degradation of precision with which the speed is estimated.
A solution to this problem is to use a widely known method of controlling the synchronous current at low speeds, in which a predetermined current vector is allowed to flow during any control phase and a synchronous phase obtained by integrating the speed command is provided as the control phase so that the speed of a motor follows speed commands. Then, when the speed command achieves a value that allows sufficient detection or estimation of the induced voltage, synchronous current control is switched to sensorless vector control (for example, see Patent Literature 1).
For a motor with magnetic salience, there is a known method of correcting the phase estimation error in sensorless vector control in a low speed region by using a method of applying a high-frequency voltage command for use in position estimation to estimate the magnetic pole position of a rotor from the detected current (a high-frequency superposition scheme), and, when the speed command or estimated speed achieves a value that allows sufficient detection or estimation of induced voltage, of migrating to the use of only sensorless vector control (for example, see Patent Literatures 2 and 3).
Additionally, high-efficiency control, such as maximum torque control with reluctance torque and maximum efficiency control that takes into consideration the core loss and the like, is widely known (for example, see Patent Literature 4 for the maximum torque control, and Patent Literature 5 for the maximum efficiency control).